Elastomeric articles, such as gloves, are well known in the industrial and medical fields for their ability to form a protective chemical, microbial, and physical barrier between the external environment and the user's skin. Various physical and chemical properties of gloves, and the elastomers they are made from, are desirable for a variety of applications. In certain fields where articles such as gloves and finger cots are widely used, such as electronic assembly, desirable properties include comfort and softness of feel, maintenance of tactile sensitivity and surface static resistivity.
The advantages of elastomeric articles such as gloves with antistatic properties are also known. In certain fields such as handling sensitive electronic equipment, the use of gloves and/or finger cots with antistatic properties is mandatory to avoid static discharge that can damage electronic components. Polyvinyl chloride (PVC), acrylonitrile-butadiene (nitrile) and polyurethane rubbers have been used for antistatic gloves.
The use of natural rubber is associated with good softness and tactile sensitivity properties. However, natural rubber does not exhibit good static resistivity. To compensate for this deficiency, conductive carbon black could be added to natural rubber latex. However, a concern with such a black-colored article is shedding of the conductive black particles which can cause contamination problems. Moreover, carbon black would darken and blacken the resulting article which is aesthetically unappealing and potentially visually distracting to the user and may interfere with the precision of handling instruments.
U.S. Pat. No. 6,794,475 to Bialke et al. (hereinafter “Bialke et al.”) describes antistatic polymers, blends and articles. Bialke et al. describes polymeric blends containing a macromer-modified latex and a secondary latex. The macromer-modified latex may be an acrylonitrile-based co-polymer and the secondary latex may be natural rubber. Articles formed from these polymeric blends allegedly demonstrate improved electrostatic properties. Specifically, Bialke et al. provides examples of co-polymers of methoxypolyethylene glycol methacrylate and acrylonitrile used in blended compositions containing one or more latex components having static resistive properties. However, Bialke et al. achieves antistatic properties by modifying a latex component with a macromer and thereby forming co-polymers containing macromers such as methoxypolyethylene glycol methacrylate. Bialke et al. does not describe blending unmodified latex components, specifically the combination of nitrile rubber latex with natural rubber latex. Nor does Bialke et al. recognize the antistatic properties of such a combination.
U.S. Pat. No. 5,459,880 to Sakaki et al. (hereinafter “Sakaki et al.”) describes gloves having a plurality of different types of rubbers successively laminated together. Sakaki et al. describes the lowermost layer as being a natural rubber layer and a top surface layer as being an oil-resistant rubber layer, such as a acrylonitrile-butadiene rubber layer. Each of these layers contains only one particular type of latex. Sakaki et al. does not disclose latex layers consisting of a blend of more than one type of latex (e.g., nitrile rubber latex combined with natural rubber latex). In addition, the process described by Sakaki et al. includes forming a lower layer and immersing the lower layer into an additional latex composition immediately thereafter or while the lower layer still contains water so as to improve the adhesion between the layers of different types of latex.
Problems associated with the manufacture of multilayered articles with different elastomer layers laminated to one another include chemical incompatibility resulting in delamination between layers and difficulty in controlling the amount of water in the latex gel and the degree of latex gelling. When these factors are not properly controlled, the layers may delaminate. To reduce delamination of the layers, Sakaki et al. relies on the ability of the rubber components of both layers to mix with each other at the interface so that the rubber molecules are entangled with one another. Such interfacial mixing could only occur if the first layer is not fully gelled or set (i.e., it is dependent on the degree or extent of gelling or setting of the first layer). The degree of gelling is dependent on the concentration of coagulant, the solids content of the latex, temperature and time, and is therefore difficult to control. Sakaki et al. does not describe increasing the chemical compatibility of two different layers by blending the two different types of latex prior to forming an article and thus reducing delamination.
There is a need in the field of elastomeric articles for an improved manufacturing technique which can produce a rubber-containing article that exhibits good antistatic or surface resistivity properties. There is a further need for multilayered rubber-containing articles wherein additional natural rubber layer(s) can be formed that afford the advantage of softness of feel and comfort on the skin-contacting layer while maintaining good adherence between the layers.